class_docstr: A tool to enforce non-reentrancy.
  Mostly used on DataFlow whose :meth:`get_data` is stateful,
  so that multiple instances of the iterator cannot co-exist.
code: def __enter__(self):
      self._succ = self._lock.acquire(False)
      if not self._succ:
          raise threading.ThreadError("This DataFlow is not reentrant!")
  
imports: import threading
  from abc import ABCMeta, abstractmethod
  import six
  from ..utils.utils import get_rng
label_desc: Use image node_id 2 for calling the DataFlowReentrantGuard obj's underlying member method code with example usage: obj.__enter__() without return types